X-ray fluorescence (XRF) analysis is universally recognized as a very accurate method of measuring the atomic composition and other characteristics of a sample material. This technique (and its close relatives) involve irradiating a sample area with high energy radiation, such as x-rays, gamma rays, neutrons or particle beams and observing the resulting fluorescence emitted by the sample area.
As discussed further below, certain challenges exist in applying XRF techniques to patterned surfaces having many, closely spaced heterogeneous materials—for example, semiconductor integrated circuits (ICs), flat panel displays, surface acoustic wave (SAW) devices, printed circuit boards, planar lightwave circuits, etc. During IC fabrication, many complicated processes are used to deposit and pattern many differing materials on a wafer. XRF can assist in monitoring certain material characteristics, for example, the thickness of deposited films. However, the extremely small feature sizes in chip regions of the IC are difficult to measure directly with XRF. XRF systems have excitation beam sizes much larger than certain feature sizes in use now, and those planned for the future. The present invention is directed to improved systems and techniques which overcome these challenges and apply the power and accuracy of XRF measurements to these applications.
XRF systems generally include a source of excitation radiation, an optic for directing the radiation toward a sample, a radiation detector to detect the stimulated fluorescence emissions from the sample (possibly through another optic), and a display of the spectral output. As the excitation photons strike the sample, they knock electrons out of their orbits around the nuclei of the atoms in the sample, creating vacancies that destabilize the atoms. The atoms stabilize when electrons from the outer orbits are transferred to the inner orbits. These atoms emit a characteristic x-ray fluorescence photon representing the difference between the two binding energies of the corresponding orbits. The detector collects this spectrum of photons and converts them to electrical impulses proportional to the energies of the various x-rays in the sample's spectrum. Since each element has a different and identifiable x-ray fluorescence signature, an operator can determine the presence and concentration of the element(s) within the sample by reviewing specific areas of the emitted spectrum.
The excitation spectra can be intentionally narrowed to a specific, “monochromatic” range. This will lower background noise from adjacent radiation bands, enabling a particular concentration of a known material to be measured. For example, the thickness of a layer of known material can be determined with monochromatic radiation tuned to the material's known fluorescence spectrum. This is accomplished, for example, using monochromating optical element(s) in the excitation path.
Patterned surfaces such as integrated circuits (ICs), flat panel displays, surface acoustic wave (SAW) devices, printed circuit boards, planar lightwave circuits, etc. present special analysis challenges because they include many layers of different materials. IC materials include the semiconductors themselves (e.g., silicon), the various insulating layers (e.g., oxides) and the metallic materials forming electrical interconnect lines or barrier layers (e.g., titanium or tantalum films). Feature characteristics, i.e., the thickness of a metallic film, can be measured using XRF techniques. And because the small feature sizes of IC features require great precision of the various processes used (deposition, etching, implantation, etc.), XRF measurements also enable accurate monitoring of these processes.
Accurate XRF techniques in these applications generally require a constant x-ray flux on the sample line itself, and detection of fluorescence attributable only to a calibrated line width of sample material. Flux directed toward other lines, and the resultant fluorescence emitted from those lines, may confuse the results. Alternatively, if other sample regions must fall in the beam footprint, the consolidated “coverage ratio” of all such regions should be constant and calibrated into the system—necessitating very accurate alignment and movement during measurement. In the IC chip regions, however, many different materials of small sizes are spaced by very small distances. This will affect the accuracy of an XRF measurement directed to a particular sample material. For example, interconnect lines or barrier layers can have sub-micron line widths in the chip regions. These widths will only decrease with time and advances in technology. It is difficult to narrow an x-ray beam to such widths, without stimulating other adjacent regions and confusing the XRF results. Alternatively, if the system is calibrated to a certain coverage ratio of sample material in the beam footprint for narrower lines, careful alignment and movement is required of the system during measurement to maintain the coverage ratio, and thus the integrity of the calibrated and measured values. Therefore, it is important to closely control the excitation beam spot size, and also to collect most if not all of the fluorescence emitted from the sample material itself for accurate XRF results.
Certain techniques may improve analysis of films deposited during IC fabrication. For example, sacrificial test wafers can be used. The film material can be deposited over large areas—with no other materials near an XRF sample area. Comparatively large sample areas can therefore be made available for XRF measurements of film thickness. However, this technique assumes that measurements made on the test wafer will “predict” the dimensions of the film deposited over the final wafer. Considering all of the variables in IC deposition and etch processes, this may not be a valid assumption. Moreover, this technique incurs the time and expense of processing an extra test wafer.
Therefore, improved techniques are required for analysis of small, patterned features, while exploiting the benefits of well-known measurement techniques (e.g., XRF) normally used for larger sample areas in other applications.